Compensated two-channel servo system



Kug. 17, 1954 m NORTON 2,686,897

CQMPENSATED TWO-CHANNEL SERVO SYSTEM Filed Dec. 30, 1949 3 Sheets-Sheetl Iowdl ATTORNEY INVENTQR glfarlan Aug. 17, 1954 NORTON 2,686,897COMPENSATED TWO-CHANNEL SERVO SYSTEM Filed D80. 30, 1949 3 Sheets-Sheet2 INVENTOR ATTORNEY iMrwo/rx 1954 L. E. NORTON COMPENSATED TWO-CHANNELSERVO SYSTEM 3 Sheets-Sheet 3 Filed Dec. 30. 1949 INVENTOR wellgJYflrlan7 .uafz/a,

ATTORNEY 1 Patented Aug. 17, 1954 Lowell E. Norton, Princeton, N. J.,assignor to Radio Corporation of Delaware of America, a corporationApplication December, 30, 1949, Serial No. 135,857

This invention relates to dual-channel servo systems such as utilized,for example, for frequency-stabilization of oscillators and particularlyconcerns arrangements for minimizing shift in the control point orset-point frequency due to differential changes in thetwo channels ofthe servo system.

In copending applications including Serial Nos.

115,698; 122,988 and 119,119, now, respectively, U. S. Patent Nos.2,631,269; 2,560,365; and 2,555,150; and in applications Serial Nos.29,836, now abandoned, and 4,497 there are disclosedfrequency-stabilizing systems in which two trains of pulses containingfrequency-error information are applied to input circuits of, aphase-comparator or time-coincidence detector to produce aunidirectional output voltage varying in sense and magnitude with thephase relation of the pulses. inthe servo system, thecarrier-frequencyof the controlled oscillator can be rigidly stabilized;within very narrow limits: however, as the circuit parametersin the twochannels may differentially change with time, ambient temperature oroperating conditions, the output frequency will shift because theaforesaid differential changes affect the time or phase relation of thepulses indistinguishable by the detector from changes in phase due tofrequency-deviation ofthe oscillator.

In accordance with the present invention, the connections of one of theinput circuits of the comparator is intermittently reversed concur-,rently with reversal of the poling of reectifiers in thephase-comparator, so that without any change in poling of theoutputvoltage, its magnitude is subject to an incremental change in sense andmagnitude corresponding with any dissimilarity between the circuitcomponents so transposed. More particularly, the incremental controlvoltage changes so produced are compared with a reference voltageproduced by or time d with the switching operations and b the algebraicsum of thesevoltages, or a voltage proportional thereto, is introducedinto the comparator network as rectifier bias automatically to reset ormaintain the control point or output frequency determined by the servosystem.

The invention further resides in methods and For a more detailedunderstanding of the invention, reference is made to the accompanylatorsystem;

By recourse to high gain or amplification I systems having the featuresof combination and arrangement hereinafter described and claimed.

8 Claims. (Cl. 324-87) Fig. 2 schematically illustrates aphase-comparator or coincidence-detector utilizable in Fig. 1;

Figs. 3A' and 3B are explanatory figures referred to in discussion ofFigs. 1 and 2;

Fig. 4 schematically illustrates a dual-channel servo system utilizablein the system of Fig. 1;

Figs. 5A, 5B and 5C are explanatory figures referred to in discussion ofFig. 4; and

Fig. 6 illustrates a modification of switching mechanism shown, in Fig.4. Referring to Fig. 1, the oscillator lit to be stabilized at afrequency F0 may supply high-frequency power to a load II through. asuitable transmission line I2. For purpose of explanation, it will beassumed that oscillator I0 is a microwave oscillator and that it isdesired to stabilize its frequency with respect to a standardtfrequencyFs which may be that at which a gas exhibits molecular resonance.

The frequency of a second oscillator I3 is repeatedly swept over a rangeof frequencies including the standard frequency. Part of the output ofthe search oscillator I3 is transmitted through line I4 to a cell I5containing, for example, ammonia, at suitably low pressure to exhibitsharp molecular resonance at the standard frequency. The microwaveenergy transmitted by the gas is impressed upon a demodulator I6 whoseoutput therefor is a series of pulses each occurringas thesearch-frequency passes through the resonant frequency of the gas.

Part-of the output of each of the oscillators ID and I3 is impressedupon a suitable mixer I9, suitable directional couplers 20 beingprovided for that purpose. The output of the mixer I9, therefore,comprises a periodically varying beatfrequency equal to the differencebetween the frequencies of oscillators in and I3. Assuming, for example,that the frequency of oscillator I0 is nominally 23.900 kilomegacyclesand that the frequency Of oscillator I 3 is repeatedly swept over therange from 23.867 to 23.872 kilomegacycles, the difference-frequencyoutput of mixer l9 cyclically varies over the range of from 33 to 28megacycles. The repetition or sweep rate of the oscillator I3 isrelatively low, for example kilocycles, but is substantially higher thanthe frequency of any modulation applied to oscillator II) fortransmission. of intelligence at audio or video frequencies.

The beat-frequency output of the demodulator I9 is impressed upon afrequency-selective network. 2| including or associated with a rectifieror demodulator 22 which like demodulator I6 and mixer I9 may be of thecrystal type. The

network 2| may include sharply tuned circuits or other types of filters,such as disclosed in the aforesaid applications. Generically in all ofthem, the output of the demodulator 22 is a series of pulses containinginformation concerning the precise time in the sweep cycle at which thebeat-frequency passes through a. prechosen value Fir.

As more fully set forth in the aforesaid applications, the two trains ofpulses, one produced by impressing the search-frequency upon. the gascell I or equivalent and the other by impressing the periodicallyvarying beat-frequency upon the frequency-selective network 2 I, areimpressed, after suitable amplification and shaping by networks I! and23, upon a phase-comparator N3 of suitable type. Also as therein.explained, the output of the phase-comparator is a unidirectionalvoltage which varies in sense and magnitude with variations in the timerelations between the corresponding pulses of the two trains. Thecontrol voltage produced is applied ta the oscillator or a tubeassociated therewith to stabilize its output frequency at the frequencywhich corresponds with the standard-frequency F'splus or minus theprechosen beat-frequencyFsselecti'vely passed by network 21 The systemof Fig. I thus briefly described is typical of a two-channel servosystem whose output on correction-voitage, under ideal conditions, isdetermined solely by the phase relation of the input voltages applied tothe phase-comparator. Actually and inevitably, there arisedi'ssimilarities in the two channels, such, for example, those due tothe effectsof temperature and other ambient conditions upon the circuitcomponents including condensers, resistors and tubes of thephasecomparator. Inconsequence, zerooutput of the phase-comparator mayoccur not at the expected coincidence of the two trains of pulses but ata somewhat earlier or later time, with the result that the oscillator isstabilized at a frequency which is somewhat displaced from the desiredoutput frequency.

In accordance with the present invention, the connections from one ofthe channels, specifically channel A, to the phase-comparator [8, may bereversed, as indicated by switch 60, Fig. 1, so that for one position ofthe switch the servo system tends to stabilize the oscillator frequencyat or somewhat above the desired frequency, whereas for the otherposition of the switch, the servo system will tendto stabilize theoscillator-frequency at or somewhat lower than the desired frequencydepending upon the sense of the net diiierential changes. Thus, byreversing the input connections at suitably short intervals, theintegrated frequency-error may be reduced. The foregoing presupposes, aslater more fully discussed, that provision is made to reverse the polingof rectifiers included in the phase-oomparator concurrently with theaforesaid switching so that the incremental change in output-voltage ofthe comparator corresponds only with the dissimilarities between the twotransmission paths in channel A and does not depend upon thefrequency-deviation of oscillator l0. Such periodic reversal is itselfof value, but alone does not provide for an absolute output frequency.To attain that end, the incremental change of output occurring upon.switching of the input channel is. reintroduced into the"phase-comparator network in compensation for the differential changes inchannel constants or operating parameters, so automatically to maintainthe proper set-point frequency despite dissimilarities between channelsof the servo system.

Although the invention is applicable to phasecomparators of manydifferent types, the one shown in Fig. 2 is selected for description ofa specific example and also to give a clearer understanding of theforegoing statements. The output pulses of the frequency selectivenetwork 2| are converted to pulses of sawtooth waveform PB applied tothe input terminal 24 of the phasecomparator L8A.. The path within thecom: parator from input terminal 24 to the anode of the upper rectifier21 includes a resistor 25 and a blocking condenser 26; the path frominput terminal' 26 to the cathode of the other rectifier 21 includes asimilar series-arrangement of a resister 2.5 and a condenser 26. One ofthe output terminals 28 of the phase-comparator WA is connected betweencathode and anode of the upper and lower rectifiers 21 respectively; theother output: terminal 30s is the. mid-point; or: common terminal 0tequal resistors: 29; connected to the two rectifiers. The outputterminals of the phase-comparator [8A are: connected to are"-sistance-capacitance. network. 32. which: smooths the pulsed output ofthe phase-comparator to provide the frequency control voltage Em. Asindicated in Fig. 2,. the pulses PB: are applied in phase (push-push) tothe .two. rectifiers2'l which, so far as these pulses are concerned,areopposite ly poled, the upper rectifier 2T conducting for positive peaksof pulses PB- and. the lower rectifier' 21- conducting. for negativepeaksof those pulses.

The pulses of channel A areconverted. as later specifically described topairs of positive. and negative pulses: (+Pa1=PA')- which are applied inpush-pull through blocking" condensers 3t, M

to the rectifiers 21,. 2"! in series-aiding relatierr.

The output of the phase-comparator network MBA is zero when, asindicated in Figs. 3A and 3B, the push-pull pulse pairs of channel occuror are at peak value when the trailing edge of the sawtooth voltage PBpasses through its mean value. Under such circumstance, the currentpulses passed by the rectifiers in each cycle are equal in magnitude andopposite in polarity so that their average value is zero andconsequently there is no change of the control voltage Ec of theintegrating network 32* for the compaartor. Zero output of thephase-comparator per sweep cycle of oscillator l3 should exist onlyunder the circumstance that the frequency of oscillator 10 preciselycorresponds. with the sum of the standard frequency Fs plus or minusthepass-frequency F1; of network 2|, depending upon which output frequencyis desired. Should, however, the positive and negative pulses (+PA-Pa)be not quite alike in amplitude, due to differential impedancevariations in the associated networks or to any other cause, or shouldthe twodiodes 2'1, 21 have slightly different contact potentials, thenzero output of the phase-detector occurs not at the expected coincidencecondition but at a point displaced in time or phase therefrom, the senseof the displacement, depending upon which of the pulses is of greateramplitude. These are but two examples of many dissimilarities' whose neteffect may cause an error in the output frequency of thefrequency-stabilizing servo system.

The phase-comparator ISB of Fig. 4 is the same as thatof Fig". 2' exceptin novel respects specifically discussed. The. connections from channelA to the input terminals 36, 36 of the phase-comparator 1813' include areversing assess? hicvable'swittahcontacts, 60in the position shown, thepositive pulses +PA are transmitted to input terminal 36A and thenegative pulses PA are impressed upon input terminal 363. When thecontacts 60, 6!] are moved out of engagement withcontacts 6|, 6| andinto engage- 'mentwith contacts62, 62, the positive pulses are impressedupon input terminal 36B 'andthe negative pulses upon input terminal 36A.Concurrently with this transposition of the conductors of channel A, themovable contacts 63, 63 of a second switch move from enga'gementwith-contacts 64 and into engagement *with contacts 65 to reverse thepoling of rectifier 21A, and the contacts 66, 66 of a third--switchsimilarlycoact with fixed contacts .61, 68 to reverse the polingof the other rectifier 21B. 'Ihus, lupon each, actuation of thereversing switches, the paths of channels through the comparatorofthepush-pull pulses +PA, -PA are transposed. If the same phase relationsof the pulses exist for both positions of thereversing switches, theincrementalchange in the output voltage of the rectifier is Zero,whereas if there are dissimilarities between the two channels. theswitching operations will cause incremental voltage changes to valuesabove or belowthe value corresponding with the proper output frequency,"all as indicatedby the curve AEc of Figs. 5A, 5B and5C. i "In theparticular arrangement shown in Fig. 4. the reversing switches areperiodically actuated by an electromagnetic relay device 70 undercontrol of a timing switch H which may comprise -motor-driven cams 12and contacts 13 actuated therebytocomplete a circuit through battery14,o'r equivalentsource of current. The reversals are effectedatsuitably short intervals, for examplaevery one or two seconds. 1KFondetecting the sense and magnitudeof "the 'incremental voltagechanges incident to switching of the channel connections,-there isrequired a sourceof reference, which in the arrangement shown in Fig. 4is provided by or 'derivedfrom the switching.

Specifically, the switches 13, 13 also control flow of current through aresistance network 15 havinginputterminals 16, 71 respectively connectedthrough resistor 84 andswitch l3to opposite terminals of thebattery 14;The input terminals 16, 11 are also connected through equal resistors83, 83to the center tap of battery Hwhich isat ground or chassispotential: The "input'terminals 16,71 are also connected through "equalresistors 85, 85 to the output terminal 80 of, the network 15. c c

The output voltage of the phase-comparator network I8B is applied totheinput terminals 18, 19 of the resistance network 15,.the resistors82,

82 connected from terminals 18 to each of terminals16,"|1 being of equalvalue. "Theoutput .voltage of the resistance network 15 is impressedupon a network comprising the resistor 86 and ?condenser 81 in shuntthereto. Thetime constant of 86; 8'! isonly large enough at theswitching period to. prevent overshoot switching transients fromappearing at amplifier 8| input. The resistors .82, 84 are of equalmagnitude, which is large compared to the resistance of each :oftheequal resistors 83,13. The magnitudeof sense, Fig. 5B, the peak value ofsum Ea of the voltages Eb and AE'caS measured slightly smoothed by 81 toprevent overshoot switching transients form appearing in amplifier 8iinput and applied to a direct-current amplifier 81 whose outputresistance 69 output frequency of mains the same despite differentialchanges in pages 300 to 301 of Handbook -Theseincremental voltagechangesmay be posi-- tive, in a phase sense as negative (in the oppositephase) as indicated in Fig. 5C. The successive closures of switches 13produce two intermittent voltages Eb and'Ea, which are of oppositesenseor phase and of equal magnitude and jointly serve as a referenceacross resistors 83, 83 of network 75. Assumingthe incrementalcomparator output pulses are or one the algebraic indicated in Fig. 5B,"or

at output terminal 19, of network 15is greater than thealgebraic sum Erof thevvoltages Ed andAEo as independently measured at the same outputterminals and consequently the difference E of these sums as applied tothem tegrating RC network is positive in sense '(Fig. 5B). i

c If on the other hand, Fig. 50, the incremental voltage changes AEc arenegative-in sense, the peak value of the algebraic sum is less than thealgebraic sum Er and theoutput difference voltage Eg of network 15 isnegative in sense: V

In both cases, the output voltage E of network J5 is proportional to AEoand of corresponding sense.

The pulsing output voltageE of network 15is the averaging network 8 5,

is connected between 27,21 to provide a bias which shifts the operatingpoint ,ojf, the rectifiers in proper sense and to proper extent toreduce the incremental changes AEc of the comparator output voltage tonegligible, or zero value, Fig; 5A thus to insure thatthe thestabilizing system 'rethe rectifiers the servo-system channels forpulses'f +PA, PA. ,The incremental changes are 'thusffed back in such asense,to the rectifier circuit of tube .2 7A that such incrementalchanges aresubjstantially cancelled or compensated.

It is not necessary, of course,that the switches for reversing thechannel connections and'pol ing of the rectifiers 21, 21 be ofreciprocating type. The switches may be of the rotating type or may beelectronic. As exemplary of the former, reference is made toFig. 6, inwhich the timing solenoid 70A through a stepping mechanismintermittently rotates shaft 92 to which are attached the movablecontacts of the 'reversing switches. Each movable contact, genericallyrepresented by contact 60A of Fig. 6,

alternately engages fixed contacts, exemplified by contacts 6| A, 62A ofthe reversing switch. As

exemplary of an electronic switch which can be used for reversing thechannel connections and rectifier poling, reference may be had to TheElectronic Engineering by Batcherand Moulic, Edition 1945, published byMaple Press Go. for Electronic Development Associates. l 1 I The pulsesare applied. in channel Ato de- -modulator I6 and amplified byanamplifier .40, then differentiated by a capacitor-resistor combinationcomprising capacitor 41 and resistor 42.

Theamplifier 44- is; normally biased to be; cutoff, and this bias is;overcome only at, the positive peaks: of the. difierentiated andamplified. pulses. The amplifier 4:4; also serves as a phase. inverterto. supply: any output. of one phase from the anode circuit. throughcoupling; capacitor 45;, and an output. of the: opposite; phasedeveloped across cathode resistor 4Tb. Resistor 46a is, for the A. C.circuit, in; parallel. with the; anode: resistor, so that. the tube.current; develops equal. amplitude pulses across, the anode. and cathodeload cirwits.

channel: B. ot 4, network 2| with rectifier demodulates pulses which.are amplified in. a stage of amplification, then applied. by adifferentiating circuit comprising. capacitor 5| and resistor 52... The.differentiated; and amplipulse is. applied, to amplifier 54. throughcoupling capacitor 54. Amplifier 54 is. normally cutofl, only the peaksof the amplified differentiated signal causing conduction, in amplifier5%. The purpose of. this, as. in amplifier 44, is set. more definitelythan. the broad.- input pulses the. time of occurrence ofthe. outputpulses. From amplifier 54, the output is taken from the junction betweentwo. capacitors 55' and 56.- connected serially between the anode of theamplifier 54 vwhich has a, load resistor as. shown) The outputthereforeis of saw-tooth form as illustrated by the; wave shape adjacent the leadtojunction 24a.

Itwillbe understood that-the outputs of channels A and B are to bebrought into a desired phase: coincidence as described in. connectionwith Figs. 3A. and 3B. The effect of contact, potentials and the like tocause unbalance in the phasev detected output. from junction 30. hasalready been described in connection with Fig. 2, and the manner inwhich such unbalance and the. corresponding error is corrected hasalready been; described. Such correction is secured by the feed-backthrough amplifier 8.1- which substantially nullifies the undesiredvoltage introduced. by such unbalance.

Though. particularly suited for use in frequency-stabilizing systems ofthe type described, it shall be understood the invention is suited forfixing the control point of dual channel servo systems: used for otherpurposes.

What is claimed is:

1. The combination with a two-channel fretruancy-stabilizingservo-system one channel producing push-pull and theother push-pushsigsaid system being of type inwhich a phaseccmparator includesrectifier means producing a frequency-control voltage varying with andhaving a:v sensing responsive to the phase relation of error pulsesrespectively impressed one in push-pull and. onev in push-push by saidchannels upon said comparator, and. an arrangement for minimizingfrequency error due. tov dissimilarities between nominally similarcomponents of said channels said; arrangement comprising first switchingmeans for reversing the connections of.

one of said channels to the phase-comparator and second switching meansfor concurrently reversing the poling of said rectifier means, andmeans'for' repetitively actuating said switching means simultaneously,whereby the average value of said frequency-control voltage isinsensitive to channel unbalance.

2. The combination set forth in claim I with addition of means forcombining a reference potential with incremental changes of saidfrequency-control voltage to produce a potential of magnitude. and sensecorresponding. with the quency-error due to. aforesaidparamcterdiifferences oi the.cham1els-..

3., The combination set. forth; inclaim 2, with addition of means forintroducina said. produced potential, into. said phase-comparator insense to, reduce said incremental changes in frequency-control voltageto negligible: magnitude.

4. A phase-comparator including rectififir means and having two inputchannels: one: up.- plying; pushapul-l. and, the other push-push.signals to said rectifier means and an output, channel, a modulatornetwork: in said outputchamrel, first switching means forintermittentlyreversaing the connecticnszoi one. oi said input channelsand second switching: means tor concurrently reversing the poli-ng; oirectifier means. in said phase-comparator to apply to said, network.cremental voltage changes corresponding; with differentialdissimilarities of: said comparator circuit, components. and means for.effectively introducing the output. of said modulator net.- work intosaid phase-comparator as a. compensating bias for said rectifier means.

5. A phase-comparator as. in. claim 4- in which a, reierence voltageapplied to; said network, is varied in synchronism with the switching.

6. A phase comparator network: haying; a. pair of output terminals andtwo; pairs. of. input. terminals, meansv to, apply pushepull signals toone pair of. said, input terminals, and means. to; apply push-push.signals. to; the; other said pair of in,- put terminals, a pair ofrectifiers connected; in series aiding relationship, to receive said sinals, a pair of components, nominally but amtually havingdissimilarities, one. said component connected circuit between, onerectifier and one pair of input terminals and the other said componentin circuit similar to the. one between the other rectifier and the.other,- pairof terminals, the voltage.- between. said; output terminalsbeing of. magnitude and sense. dependent on the phase relationship ofsignals. applied to said pairs of input. terminals, and also: uponsimilarities oi said components: and rectifiers, and means forminimizingthe, eiiect of said dis.- similarities upon said outputvoltage, said mini.- mizing means comprising: switching. means forintermittently reversing the signal connections to said one pair ofinput: terminals, and switch? ing; means for reversing the poling oi;said rectifiers concurrently with the said reversing of connections tosaid input. means.

'7. Asystemas claimed claim 63, inwhich the said output connections areconnected. into the said network toreintroduce into said phasezcomparator a voltage which substantially complete,- ly compensates fordissimilarities. in said rectifiers.

8. In a. two-channel. frequency-stabilization servo-system of the typein which the phase of push-pull pulse signals from one, channel arecompared with push-push signals from the other channel in a phasecomparator which includes a pair of rectifier means connected to receivesaid signals, poled one to produce a rectification voltrectificationvoltages; an arrangement for minimizing frequency error due todissimilarities between nominally similar components of said channels,said arrangement comprising first switching means for reversing theconnections of said one channel to apply said push-pull signals inreverse polarity, second switching means to reverse-the poling of saidrectifier means, and means for simultaneously and repetitivelyactuatingsaid switching means.

References Cited in the file of this patent Number UNITED STATES PATENTSName Date Bowen Sept. 21, 1937 Zuschlag June 30, 1942 Ziegler et a1.Aug. 20, 1946 Learned Mar. 22, 1949 Rich May 1, 1951

